The present invention generally relates to optical sources, and more particularly to a single-photon generator and single-photon generation method.
Safe and secure cryptographic telecommunication is indispensable technology for realizing the next generation information society including electronic government, electronic commerce, and the like.
Currently, the RSA public key cryptosystem is used extensively in the Internet for providing security on communications exchanged through the Internet. According to the RSA public key cryptosystem, the security is provided solely on the principle of difficulty of calculating the factorization of polynomials into prime numbers and thus relies on the difficulty of conducting huge amount of calculation in a practical time. This means, on the other hand, that when a quantum computer, capable of conducting parallel processing at enormously high speed is realized, the time needed for decrypting a code is shortened drastically, and it is no longer possible to prevent copying, such as wiretapping or falsification, of data by a third party when public key cryptosystem is used.
Quantum cryptography is expected to provide solution to this problem of security.
In quantum cryptography, information is transmitted not in the form of conventional aggregate or bunch of photons but in the form of single photons.
In the case one bit of information is imparted to a single photon in the form of polarization information, for example, the information thus given to each photon complies with the uncertainty principle and no-cloning theorem, and because of this, it becomes no longer possible to take out the bit information without destroying the state of the photon. Thus, in the case a third party has conducted copying or falsification of the information in the communication path, such a conduct is detected immediately.
Thus, in the case of using quantum cryptography, the security of cryptographic key shared between two parties is guaranteed by the physical principle, not by the quantity of calculation, as long as a single photon is used for the carrier of the information. On the other hand, in order to secure the security of such a cryptographic key and to eliminate the risk of wiretapping, it is necessary to provide a highly reliably single-photon source.
In a conventional quantum cryptography experiment, a diminished laser beam pulse has been used as a quasi-single-photon source (Rev. Mod. Phys., vol. 74, 145 (2002)). With this technology, the probability of existence of photons is reduced to the level of about 1 photon per 10 pulses in average, by diminishing the laser beam power by using an attenuator. In this technology, it becomes possible to achieve various advantages such as simple system setup, capability of changing the wavelength, capability of operation at room temperature, and the like.
In this conventional technology, on the other hand, because of the fact that the timing of emission of single photon complies with the Poisson distribution, there can arise the situations with finite probability that no photon emission occurs at all or several photons are emitted simultaneously. Thus, such a conventional system has a drawback in that efficiency of photoemission is poor. Further, such a conventional system has a problem of vulnerability to attack by a third party. More specifically, the aforementioned security of quantum cryptography is guaranteed only for the case in which there occurs single-photon emission, as noted previously. This security pertinent to the quantum cryptography vanishes when there are formed several photons simultaneously.
There are known mechanisms of single-photon generation that use the principles different from the one that attenuates a laser beam.
For example, Kim, et al., reports a single-photon turn-style device that generates a single photon for each external modulation electric field, by using the effect of Coulomb blockade occurring at a pn junction of semiconductor (Nature, vol. 397, 500 (1999)). Further, Lounis et al., report that single photons can be generated with repetition of 6.25 MHz at the maximum, by embedding molecules of terrylene into a thin film of p-terphenyl (Nature, vol. 407, 491 (2000)).
However, the former proposal has a drawback in that the operational temperature is extremely low (50 mK), and there arises a problem associated with this that unnecessary photons are formed by the background current. In the latter proposal, on the other hand, p-terphenyl in the matrix produces unnecessary photons. Thus, none of these conventional proposals could provide the solution leading to practical quantum cryptography device.
It should be noted that next two requirements are imposed for a single-photon optical source used for a quantum cryptography device.
{circle around (1)} There should occur no simultaneous emission of plural photons (anti-bunching)
{circle around (2)} Capability of emitting a single photon with desired timing (photon on-demand)
Conventionally, there have been problems in any of {circle around (1)} and {circle around (2)} or in both of these. If the requirement {circle around (1)} is not satisfied, there would occur increase in the probability of overlooking wiretapping. When the requirement {circle around (2)} is not satisfied, on the other hand, the error at the reception side is increased and sufficient bit rate is not achieved.